fix(media): don't upscale small images in the detail previews

The detail page, media manager and media diff requested a bounding-box
resize at the full box size, so an image smaller than the box was

fix(media): don't upscale small images in the detail previews

The detail page, media manager and media diff requested a bounding-box
resize at the full box size, so an image smaller than the box was enlarged
and fetch.php generated and cached the upscaled copy.

slika 1.2 adds an $upscale option to resize/crop. getDisplayDimensions()
now takes a $fit flag mirroring fetch.php one to one and predicts the
matching no-upscale dimensions, media_resize_image()/media_mod_image()
forward $upscale, and fetch.php disables upscaling for fit=1 requests.
In-page image scaling is unchanged.

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